Sausage casing is made out of either animal intestines or an artificially processed material, such as collagen, cellulose or plastic. The most common kind of plastic is nylon, though polyethylene and polypropylene are also used.
The oldest known sausage recipes utilized intestines from lamb and goats. Cow and pig intestines are made into casings as well. Traditionally, only a specific layer of the small intestine is used to make edible casings. This layer is called the sub-mucosa and is especially rich in collagen. Other parts of the intestine may be used to make non-edible natural casings. These include the large intestine and the small intestine, which does not have the muscular layer removed.
Artificial collagen casings are not made from sub-mucosal collagen but from highly processed hide and bones. Cellulose for sausage casings often comes from wood pulp and cotton producers. Plastic for casings is made via a similar process as other plastics.